In the manufacture of frames for aircraft and space vehicles there are very heavy demands placed upon fastening systems. The relatively simple task of securing a nut upon the end of the bolt must be accomplished thousands of times. The consequences of inefficiency in the manufacturing process are significant, and anything less than optimum performance of the completed structure can have disastrous effects.
When the nut is placed upon the extreme threaded end of the bolt it is first necessary to properly engage the threads of the nut with the threads on the bolt. Then the nut can be rotated quite rapidly for some number of revolutions because it is in "free" rotation--that is, its rotation is opposed only by the friction between the two sets of threads. This friction produces only a rather low level of counter-torque.
The next occurrence is that the nut comes into contact with the surface of the aircraft frame or other body to which the fastening system is being applied. As soon as this happens the counter-torque increases rapidly, because the end face of the nut is now in frictional contact with that surface. After the initial contact, when the nut is tightened even a portion of a revolution, since the effective length of the bolt is being reduced, an axial stress is developed. This axial stress increases continuously as the nut is being tightened. The greater this stress the greater is the friction counter-torque created at the end face of the bolt in contact with the surface of the body.
A problem that has occurred in the past is that the nut is rotated rapidly during the "free rotation" step, the nut and its driving tool acquire a great deal of inertia, and the result is that upon impact with the surface of the body some structural damage is caused. This structural damage may be to the threads of the bolt or the threads of the nut but is not necessarily thus limited. Therefore, careful control of the "free" rotation impact is extremely desirable.
One present method of assembly has been to perform the entire operation with a hand tool. The "free" rotation is then adequately controlled but the process is slow and is expensive in terms of labor.
Another present method has been to use a power tool that is capable not only of driving the nut during its "free" rotation, but also of tightening the nut after contact with the surface of the structural body is achieved. This approach is inefficient and is likely to cause structural damage.
Another requirement of the installation procedure is that the nut be tightened to an extent which will produce exactly the axial tension inside the bolt that the design specifies. An established method of controlling the axial tension inside the bolt is to tighten the nut only to the point where the counter-torque or reaction torque reaches a predetermined value. That predetermined value is for the most part calculated but is also in part based upon laboratory tests.
There are at present two established methods for stopping the tightening of the nut at the predetermined level of counter-torque. One method is to build a thin-walled wrench socket on the rearward end of the nut, with the thin wall being so designed that it will shear off when the predetermined torque level is reached. The other established method is to use a tool which will stop applying rotating force when the predetermined torque level is reached.
The industry has felt a need to find an instrument which will perform all three requirements--running the nut rapidly in "free" rotation with low inertia of the rotating parts; switching to lower speed and higher torque when contact is made with the surface of the structural body; and then discontinuing the application of rotating force when the desired level of torque has been reached.